1. Field of the Invention
The present invention relates to a tap which is used when a female screw is manufactured in a workpiece.
2. Prior Art
Heretofore, among taps used for manufacturing female screws in workpieces, for example, those shown in FIGS. 7 and 8, there are known taps designed for, in particular, decreasing the cutting resistance and for improving chip discharge quality. The taps shown in these figures are formed by a male screw part 2 on the crest side of the tap main body 1 forming a shaft, and are formed by a plurality of grooves 3 spiraling around the axis of tap main body 1 in this male screw part 2, and are further formed by a cutting edge 4 on the ridge part of the wall of these grooves 3 and on the outer circumference of male screw part 2; due to the spiraling cutting edge along the entire length, this type of tap is called a "spiral tap". As indicated in FIG. 9, in the spiral tap 5 in the illustration, all the circumferential pitches P on the crest of cutting edge 4 are set to be equal. In addition, the helical angles .theta. of cutting edges 4 are all set to be equal.
In the spiral tap 5, as the angle which the cutting face of a land makes with the longitudinal axis of the tap (=.theta.) of cutting edge 4 increases in the positive angle direction, the cutting resistance decreases, and since the chips discharged in grooves 3 are forced out toward the heel side of tap main body 1 by the walls of grooves 3, the efficiency of chip discharge improves.
Moreover, there are other known taps having improved chip discharge efficiency (in addition to the spiral tap 5) for example, those shown in FIGS. 10 and 11. The taps shown in these figures have in common certain features shown in FIGS. 7 and 8 in that a plurality of groove parts 6 is formed by male screw parts 2 of tap main body 1 and in that cutting edge 7 is formed by the ridge part of these groove parts 6 and on the outer circumference of male screw part 2; and they differ from tap 5 of FIGS. 7 and 8 in that the crest side walls of groove parts 6 are formed by a twisting surface spiraling in the direction opposite to that of the spiral tap 5, towards the axis of tap main body 1; spiral part 8 is formed by the crest side of cutting edge 7, and chips produced by said spiral part 8 are discharged to the crest side of the tap; this type of tap is called a "gun tap".
As indicated in FIG. 12, the circumferential pitches P of the tap crest of the cutting edge 7 are set to be mutually equal in gun tap 9. Furthermore, the angles which the cutting face of a land makes with the longitudinal axis of the tap .theta. of spiral part 8 are also set to be equal. In gun tap 9, instead of the spiral part 8, a straight notch crossing the axis of tap main body 1 at a skewed angle is cut in the crest side of groove parts 6, and therefore an inclined part at the crest side of cutting edge 7 is provided, and the angles which the cutting face of a land makes with the longitudinal axis of the tap of the crest parts of cutting edge 7 are all set to be equal.
In the above-mentioned taps 5 and 9 of the prior art, the thread formed in the workpiece by cutting edges 4 and 7, and the above male screw part 2 of taps 5 and 9, make contact and advance during processing; however, at times the cutting resistance load on cutting edges 4 and 7 changes and produces an irregular radial deflection in male screw part 2. Since the thread formed by cutting edges 4 and 7 and male screw part 2 do not come into perfect contact when such a deflection occurs, the shape of the resulting thread differs from the intended shape, production accuracy deteriorates, and the thread is defective.
Recently, due to greater control of the rotation frequency and feed rate of the tap main body 1 for which numerically controlled machine tools are used, automation and increased speed of female screw production is widely practiced. The production of female screws by such mechanical means is much faster when compared to the cutting speed attained when female screws are manufactured by manual means. Consequently, the vibration of the tap due to irregular changes of the cutting load put on cutting edges 4 and 7 easily exceeds manageable limits, manufacturing accuracy deteriorates extremely, and it is possible that damage to the tap will result.